Method and apparatus for heat recovery from drying oven effluents



June 11, 1957 w. M. BOWEN nl 2,795,054

METHO AND APPARATUS FOR HEAT RECOVERY FROM DRYING OVENA EFFLUENTS enea bot. 7, 1954 2 Sheets-sneetl o no INVENTOR;

WILLIAM M. BowEN m* ATTORNEY June 11, 1957 w. M. BOWEN m l METHOD AND APPARATUS FOR HEAT REcoyzRy FROM DRYING OVEN EFFLUENTS Filed Oct. '7, 1954 7 8 VOLUME GASES CATALYZED PER UNIT TIME VOLUME GASES EXHAUSTED PER UNIT TIME FIG. 3.

INVENToR. WILLIAM M. BOWEN III FIG. 2.

United States Patent YO METHOD AND APPARATUS FOR HEAT RECOV- ERY FROlVI DRYING OVEN EFFLUENTS William M. Bowen III, Paoli, Pa., assigner to Oxy-Satalyst, Inc., a corporation of Pennsylvania Application October 7, 1954, Serial No. 460,935

7 Claims. (Cl. 34-35) This invention relates to drying and the like ovens of the general type wherein the oven atmosphere contains an oxidizable mixture of air and a combustible gas. Ovens of the type adapted to volatilize the solvents of paints, enamels and the like in which the mixture of air and solvents is exhausted to the atmosphere are one type of oven to which the principles of the invention can be applied.

lIn a conventional drying oven of the type, for example, in which enamel, paint or ink is baked or dried, the work in the form of Wire, strip, or sheet metal, paper or cloth, travels through the oven continuously. For Wire or strip material, the work is usually pulled through the oven while in the case of plates or sheets the material is carried through the oven on an endless conveyor. The atmospheres of such ovens are maintained at a suitable drying temperature which is usually in the range of 400 F.-500 F. although higher or lower temperatures may be employed for specic operations requiring other than ordinary conditions. While passing through the oven the Isolvents contained in the enamel are vol-atilized and inter- -mix with the air in the oven to produce an air-vapor mixxture. The mixture is continuously exhausted from the 4oven by means of a lluid impeller such as a fan which fdraws the air-vapor mixture through an oven exhaust duct :and forces it through a stack to the atmosphere. In most 'cases the rate o-f exhausting gases from the oven atmosphere is rigidly controlled in accordance with the requirements of insurance underwriters for the reason that safety requirements dictate that the concentration of vapors in :the oven be maintained 4below the lower limit of explosability. If the concentration is allowed to rise to the point at which it attains the lower limit of explosability, an imminent danger of a `serious -iire or explosion arises. For :this reason the concentration of vapors in the oven atmosphere is `generally maintained at a level at which it is about i thirty percent or less of the concentration of the lower imit of explosability.

yIn conventional drying ovens, the oven atmosphere is internally recirculated continuously at a relatively high yvelocity during the drying operation. This recirculation 'is usually accomplished by withdrawing gases from the :oven atmosphere land reintroducing them into the oven `,by means of a lian. Such recirculation assures a relatively 'uniform temperature in Iall parts of the oven and thereby avoids the possibility of hot spots which impair the quality Jof the work. Furthermore, internal recirculation ma-in- .ta-ins the concentration of solvent vapors relatively uniform throughout the oven atmosphere and thereby -avoids areas of high concentration which might constitute a potential explosive hazard. 'Ihe volume of gases recirculated Ias noted above is relatively high, yand may be as much as 'ten times the volume of gases exhausted .although for some operations a recirculation rate equal to only about three times the exhaust rate may prove satisfactory.

The heat required to maintain drying ovens at their operating temperature is generally lsupplied from either liquid or gaseous fuel burners. These burners are preffice erably but not necessarily incorporated into the .recirculation system of the oven so that the rapidly moving recirculating gases are heated and immediately distributed throughout all parts of the oven. In this manner the heat required to operate the oven is uniformly distributed and undesirable hot spots are avoided. lIn conventional practice the oven burners Kare modulated by' means of controllers to maintain a constant oven temperature;Y `The amount of fuel required to operate such ovens is ingeneral lcomparatively high for the reason that the heat losses due to exhausting and to the -work itself are relatively high. For example, in addition to the usual radiation losses a conventional drying oven will lose heat by exhausting gases to `the atmosphere and will lose additional sensible heat in the work which continuously moves from the oven. For this reason `fuel costs constitute one of the major items of expense in operating these ovens.

lt has been previously proposed that the vapors contained in the exhausts from drying and the like ovens might be catalytically oxidized, and that at least a portion of the oxidized eiliuent might be returned -to the oven. This proposal presents obvious .advantages in that oxidation of the combustible vapors in the exhaust results in a considerable increase in the temperature of the exhausts so lthat reintroduction of the oxidized exhausts into the oven can result in substantial fuel savings. However, the practice of such exhaust oxidation and recirculation `schemes presents -engineering diiculties which detract from their advantages. These diiculties, which concern the `effective rate of exhausting from the oven, the heat balance of the oven, the iiow of `drying medium within the oven, and the instrumentation and ducting required, are discussed in the following para-graphs in the interest of a more thorough understanding of the present invention.

As previously mentioned, the rate of exhausting from drying ovens is rigidly controlled in order to avoid the danger of the concentration of combustible vapors attaining explosive proportions. Where, as with previously proposed systems, the exhaust gases are passed through an oxidation catalyst and subsequently recirculated into the oven, it is obvious that combustible vapors will be continually removed from the oven atmosphere only so long as the catalyst performs its intended function of effecting the oxidation of the vapors. Obviously, if the combustible components of the exhaust are entirely eliminated by catalytic oxidation, the exhausts may be introduced into the oven without danger of explosion. However, if the catalyst were to become inactive, as by poisoning or by the deposition of a coating of inert material from the exhausts, the gases reintroduced into the oven would effect an incre-ase in the concentration of combustibles in the oven latmosphere and thereby create Ian explosive hazard. Such :contingencies must be provided for in view of the hazards of drying oven operation.

One solution to the above discussed problem is to provide instrumentation ,and control mechanisms to the oven operative to detect any inactivity on Ithe part of the catalyst and to shut down the oven in the event of failure of the catalyst. This solution, however, requires relatively complex and expensive apparatus and is not readily applicable to existing oven install-ations Without exhaustive engineering ysurveys and modifications.

Where an attempt is made to modify an oven in accordance -with previously-proposed systems of catalytic exhaust treatment and recirculation of the exhausts,rit is necessary to consider the problems of heat balance of the oven. `It must -be remembered that under many circumstances catalytic oxidation of the combustible vapors in oven exhausts will result in a rtemperature rise of about 500 F. so that where the oven is maintained at a temperature of about 450 yF. the exhaust gases, after catalytic oxidation, will be at a temperature of approximately 950 F. Obviously, to rebalance the yoven on the basis of a stream of oxidized exhausts at such a temperature would require extensive and time-consuming studi-es. Furthermore, under most circumstances the temperature of the oxidized gases would necessarily have yto be reduced before introduction into the oven since a stream of gases at a temperature of 950 F. might impair the quality of the work where, Ias is usually the case, the optimum drying temperature is in the range of 400 F. to 500 F.

Finally, it should be pointed out that where a catalytic exhaust treatment and recirculation system of the presently known type is incorporated into an oven, expensive and time-consuming structural changes to the oven are required in order to provide the necessary ducting oph erative to recirculate the oxidized gases and to maintain the necessary rate of internal recirculation. Such structural changes naturally discourage modification of existing ovens.

It has been found that the above-discussed diiculties can be obviated by incorporating oxidation catalyst in the internal recirculation system of the oven in accordance with the principles of the present invention.

The invention is concerned with a drying oven in which the gases comprising the oven atmosphere are continuously internally recirculated at a relatively high rate while a predetermined volume of effluent is continuously ex hausted from the oven in order to maintain the concentration of solvent vapors at a safe level. The rate of recirculation is relatively high relative to the rate of exhausing and the rate of exhausting is maintained at a rate such that the concentration of solvent vapors will be maintainedy at a safe level regardless of the effect of the catalyst. In the practice of the invention the stream of internally recirculating gases is divided into aV minor portion and a major portion. The minor portion of this stream is passed over an oxidation catalyst to effect catalytic oxidation of the oxidizable vapors therein with concomitant liberation of their heat of oxidation. This oxidation step raises the temperature of the minor portion of the stream to a level considerably above the operating temperature of the oven. Therefore this minor portion after oxidation is not suitable for introduction directly into an oven atmosphere for the reason that hot spots would be created and thereby impair the quality of the Work. However, when the oxidized gases are intermixed with the major portion of the recirculating stream of gases a resultant gaseous mixture is obtained which is at a temperature above that of the oven atmosphere but which at the same time is not so high as to be unsuitable for introduction into the oven.

As will be apparent from the more detailed description of the invention presented hereinbelow, the present invention can be incorporated into existing ovens without disturbing either the previously existing heat balance or the flow pattern of the oven. Furthermore, the invention requires no expensive instrumentation or complex ducting. Finally, it should be noted that the practice of the invention increases the margin of safety of combustible vapor concentration in the oven and contributes materially to the abatement of the air pollution problem created by drying and the like ovens.

In the annexed drawings:

Fig. 1 is a cross-sectional view with parts broken away showing a drying oven constructed in accordance with the principles of the invention and adaptedrto operate in accordance with the method of the present invention.

Fig. 2 is a sectional View taken along the lines 2 2 of Fig. 1.

Fig. 3 is a curve illustrating one of the principles of the invention.

Fig. 4 is a perspective View with the parts broken away 4 of a catalytic unit of a type usable in the practice of the invention.

The embodiment of the invention shown in Figs. 1 and 2 is particularly adapted to an enamelling operation in which a coated strip is provided with a coating of enamel and the volatile solvents therein are volatilized with in the oven atmosphere. It is realized however that this embodiment is described by way of example only and that the principles of the invention are generally applicable to ovens in which volatile solvents are evolved.

1n the embodiment of Figs. 1 and 2 the reference numeral 1 denotes a conventional drying oven having an interior oven chamber 3. Material 4 in the form of metallic strip passes continuously through the oven in the direction of the arrows through an inlet port 6 and an outlet port 8 provided at each end of the oven. An exhaust duct 10 provided in communication with the oven chamber 3 leads to a fan 12 which draws the air-vapor mixture from the oven chamber 3 and continuously exhausts it through a stack 14 to the atmosphere. Internal recirculation of the air-vapor mixture is achieved by means of a series of ducts 15, 17, 17a, 32, and Z6. As shown by Fig. 1, the stream of recirculating gases ilows from the oven chamber into duct 16 from whence a portion of the stream iiows through duct 17 while a second portion flows through a by-pass duct 32. Duct 17 leads to a burner box 20a within which a suitable number of primary or first burners 20 are mounted. Burners 20 are supplied with fuel from a line 21, the amount of fuel supplied being regulated by a controller 22 which is responsive to a temperature sensing means 24 disposed within oven chamber 3. Duct 17a leads from burner box 20a to a fan 18 which is in turn connected to a return F duct 26 and a distributor duct 28.

Duct 32 permits the ilow of gases from duct 16 to a catalyst chamber 33 having burners 34 (herein referred to as secondary burner) and a plurality of catalytic units 40 therein. Burners 34 are supplied with fuel from a line 35, the amount of fuel supplied being controlled by a controller 36 actuated by a temperature sensing means 38 disposed downstream from the catalytic units 4). The catalytic units 40 may be of any suitable type although a particular oxidation catalyst which has been found to be particularly satisfactory in installations of the disclosed type will be described in detail below. For reasons which will be explained below, the secondary burners 34 are mounted upstream from catalytic units 40. Dampers 42, 44 disposed within ducts 17 and 32 permit adjustment of the relative flow rates of gases through burner box 20a and through catalytic chamber 33. By-pass duct 32 merges with duct 17a at a point upstream from fan 18 so that the gases flowing through this duct are intermixed with the gases flowing through burner chamber 2da.

In the operation of the embodiment shown in Fig. l the material 4 which has previously been coated with enamel passes continuously through the oven chamber 3. While in the oven the solvent contained in the enamel is volatilized and intermixes with air inthe oven atmosphere. As a result the oven atmosphere at all times is composed of a mixture of air and solvent vapors at a temperature which is usually within the range of about 400 F. to 500 F. The exhaust fan 12 continuously draws a portion of the air-vapor mixture through conduit 10 and exhausts it to the atmosphere through stack 14. At the same time the fan 18 continuously internally recirculates a large volume of air-vapor mixture through ducts 16, 17, 17a, 32, 26 and distributor duct 28. As noted above, the volume of'gases recirculated per unit time is generally much greater than the volume exhausted per unit time through exhaust duct 10. The ratioV of gases recirculated to gases exhausted may be as high as 10 to l, although in some circumstances ratios as low as 3 to l will suice to produce a satisfactoryiproduct.

p The heat required to maintain the oven at its operating temperature 'is supplied by the primary or first burners 20 and by the second burners 34 in conjunction with the catalyst 40. The major portion of the recirculating gas stream iiows through duct 17, burner box 20a and duct 17a and is heated by means of the primary or first burners 20. A minor portion of the recirculating stream ows through duct 32 and catalytic chamber 33. This minor portionof the recirculating gas stream is, of necessity, heated to a relatively high temperature by the cumulative effect of the second burners 34 and catalyst 40 for the following reasons. Practical, presently available oxidation catalysts are operative only if the reactants involved are maintained at a temperature equal to or greater than the operating temperature of the catalyst. This operating temperature will vary with different catalysts or operating conditions but in general it can be stated that the reactants should be at a temperature of 700 F. or above. Since the required oven temperatures for conventional enamelling or similar operations are in the range of 400 F. to 500 F., it is obvious the air-vapor mixture evolved must be preheated before catalytic oxidation of the solvents can be accomplished. In the practice of the instant invention the minor portion of the recirculating gas stream is heated to about 700 F. by the burner. Upon subsequent passage of the minor portion of the gases through the catalytic chamber 33 their temperature is further increased to about 950 F. or l000 F. by the heat of oxidation of the solvents. Controller 36, which is responsive to temperature sensing means 3S operates to maintain the temperature of this portion of the gas stream within relatively close limits and assures a relatively constant temperature of these gases after heating and oxidation thereof. With this arrangement the cumulative heating effect of the burners 34 and catalyst remains constant although the proportional con tributions of the burners and catalyst may vary.

The major portion of the stream of recirculating gases is heated by burners 2i) to the extent necessary to supply any additional heat required to maintain the oven at its operating temperature. These burners are controlled by controller 22 which is responsive to temperature sensing means 24 and is therefore directly responsive to the temperature of the oven. As will be demonstrated below, the burners 20 may supply only a small portion of the total heat required to maintain the oven at its operating temperature. The temperature rise of the major portion of the recirculating'gases effected by burners 20 is relatively srnall by comparison with the temperature rise in the minor portion of the gas stream. As will also be dernonstrated below, this temperature rise may be less than F.

Upon intermixing of the minor portion of heated and catalyzed gases (from conduit 32) with the major portion of recirculating gases, a resultant stream is produced which is at a temperature suitable for introduction into the oven. As previously mentioned, hot spots in a drying oven can seriously impair the quality of the finished product. In the operation of the drying ovens it is therefore preferable to supply the required heat by raising a relatively large volume of recirculating oven gases through only a relatively small temperature differential rather than by raising the temperature of a small volume of recirculating gases through a large differential. In the practice of the present invention the cumulative eifect of the burners 34, the burners and the catalyst 40 is to raise the temperature of the total volume of recirculating gases through a suitably limited temperature difieren tial as will be also shown by example below.

A salient feature of the invention concerns the relative rates of flow of gases through the recirculation duct 17 and the by-pass duct 32. As previously mentioned, the major portion of the recirculatinig stream iiows through the duct 17 and only a minor portion of the gases ow through duct 32 and across the catalyst 40. The signiticance of this aspect of the invention stems from the :fact that the volume of gases catalyzed in the practice of the invention is limited (l) by the heat requirements of the oven and (2) for economic reasons by the amount of catalyst required. The latter factor will be treated in more detail below. With reference to the heat requirements of the oven it is obvious that the amount of heat supplied to the oven must not be in excess of the amount required to maintain the oven at its operating temperature. To supply an excess of heat to the oven would tend to increase the oven temperature above the optimum operating temperature and thereby impair the quality of the work.

As noted above, the vapor laden gases which are oxidized at the catalyst 40 must in most cases be pre-heated by the burners 34 in order to render them amenable to the oxidation process. As a result, a substantial amount of heat will be supplied to the oven by this burner during the process. Furthermore, the amount of heat supplied by the burners 34 will be directly proportional to the rate of ow of the gases over the catalyst and will increase if the rate of ow over the catalyst is increased. From these considerations, it follows that for a given set of operating conditions, the rate of flow of gases over the catalyst (or the amount of heat supplied to the oven from catalytic oxidation) must be limited in such manner that the total heat supplied by the preheating of these gases plus the heat liberated by catalytic oxidation will not exceed the requirements of the oven. As will be shown by example hereinbelow, for an oven operating under ordinary conditions (as regards temperature and vapor concentration in the oven gases), this factor will limit the rate of ow of gases over the catalyst to approximately the rate of exhausting from the oven.

The amount of catalyst required to recover the heat of oxidation of the vapor-laden gases must also be taken into consideration in determining the rate of liow of gases over the catalyst. Before considering the significance of this factor, however, it must be realized that the maximum amount of chemical heat theoretically recoverable from the exhaust gases produced in the operation of Va drying oven is determined not by the volume of gases which tiow across the catalyst but rather by the ratio of the volume of gases which flow through the catalyst per unit time to the volume of gases exhausted per unit time. This fact can readily be appreciated from a consideration of the following conditions of operation of the disclosed preferred embodiment of the invention.

The strip 4 will, for a given set of conditions, volatilize a relatively constant amount of solvent during its travel through the oven. In other words, for a constant rate of travel of work, a relatively constant volume of vapor will be evolved in the oven per unit time. In conventional oven operation this vapor is dissipated `entirely through the exhaust duct 10 and the rate of exhausting from the oven will therefore determine the concentration of vapor in the oven atmosphere under operating conditions. With the present invention, however, the solvent which is vaporized is dissipated not only by exhausting, but also by oxidation since catalytic oxidation destroys substantially all of the solvent contained in the gases which ow over the catalyst. In so far as vapor concentration of the oven atmosphere is concerned, owing the recirculated gases through the catalyst 40 has the same effect as exhausting these gases through the stack 14. Therefore, for a constant rate of exhausting and a constant rate of solvent volatilization, the catalyst 40 will reduce the con-v centration of vapors in the oven atmosphere below the equilibrium level which would exist without use of the catalyst. For example, if it is assumed that the volume of gases flowing in by-pass conduit 32 and across catalyst 40 is equal to the volume of gases exhausted throughduct 10, then the equilibrium concentration of vapors in the oven atmosphere will be about one-half of the concentration which would exist in the absence of the catalyst (assuming that initially the concentration of vapor in the exhaust was equal to the concentrationof vapors in the recirculating gases). Under these circumstances one-half of the total solvent vaporized in the oven will be dissipated through the stack to the atmosphere while the other half will be dissipated by catalytic oxidation in the catalytic chamber 33. It follows then that Where the above conditions obtain (i. e., if volume of catalyzed gases equals volume of exhausted gases) a maximum of fifty percent of the chemical heat of the volatilized solvents can theoretically be recovered by the catalyst and utilized to heat the oven chamber. By the saine reasoning process it is apparent that if the volume of gases catalyzed per unit time is twice as great as the volume exhausted, two-thirds of the chemical heat of the volatilized solvent can theoretically be recovered and the other third will be lost up the stack 14. This principle is clearly illustrated by Fig. 3 wherein the abscissa represents the ratio of the volume of gases catalytically oxidized per unit time to the volume of gases exhausted per unit time. The ordinate of Fig. 3 represents the percentage of the heat of oxidation of the volatilized solvent which can `theoretically be recovered. From this curve it is evident that where a ratio is 1:1 fifty percent of the heat content of the volatilized solvent can be rccovered; where the ratio is 4:1 (i. e., four times the volume of gases are sent through the catalyst as are exhausted through the stack) eighty percent of the heat content of the solvent can be recovered. In other words, if the volume of gases which flow through the by-pass conduit 32 is multiplied by a factor of 4, the amount of heat which can be recovered is increased only from titty percent to eighty percent of the theoretical maximum.

It should be mentioned that the curve of Fig. 3 assumes that the concentration of solvents in the gases which are exhausted is equal to the concentration of solvents in the gases which are catalyzed. lf it is assumed that the concentrations of solvents in the oxidized gases and in the `exhausted gases are not equal, the curve will be of the general form shown in Fig. 3 but will have a somewhat different slope. Therefore, the general principles set forth above regarding the amount of heat theoretically recoverable from the recirculated gases by catalytic oxidation apply where the concentration of solvents in the exhausts is higher or lower than the concentration of solvents in the gases which are catalyzed although the precise values recoverable for a given ratio might be somewhat different than the values shown in Fig. 3.

With reference to the present invention the phenomenon discussed in the above paragraphs must be considered in relation to the amount of catalyst required to recover the theoretically recoverable heat in the recirculated gases. Under given conditions the amount of catalyst requiredto eciently oxidize the vapors contained in these gases will be directly proportional to the rate of ow of the recirculated gas stream. For example, the amount of catalyst required for a gas flow of 10,000 S. C. F. M. (cubic feet per minute reduced to standard conditions of temperature and pressure) will in general be ten times the amount required for a gas ilow of 1000 S. C. F. M. assuming a constant inlet temperature. Furthermore, the amount of catalyst required for a stream of gases of constant flow rate is relatively independent of' the concentration of combustibles in the gas stream. The amount of catalyst required does vary slightly with concentration of combustibles in the stream but this'variation is relatively insiguicant, particularly (as with drying ovens operated at a constant temperature) if theinlet temperature is held constant.

From the foregoing discussion it is apparent that if, in the operation of an internally recirculating drying oven, operated at a constant temperature, it is desired to catalytically oxidize the vapors contained in the recirculating gases, the amount of catalyst required for the operation vwill be directly proportional to the rate `of re- Volume of gases through catalyst Volume of gases through catalyst-i-volume of gases exhausted where H represents the heat oxidation of all of the solvents vaporized. Fig. 3 illustrates this relationship since the abcissa is an indication of the amount of catalyst required to recover a given percentage of the heat of oxidation, H, of the solvent. Thus it can be seen that if one unit of catalyst (i. e., one catalytic unit of a type described below) is required to recover 0.5 H, four units of catalyst will be required to recover 0.8 H and nine units of catalyst will be required to recover 0.9 H.

From the foregoing discussion it is believed to be evident that a process of catalytic recovery of the heat of oxidation of solvent vapors evolved in the operation of a recirculating drying oven requires that the amount of gases catalyzed be limited in the light of the heat requirements of the oven and the economic use of the catalyst. With the present invention these conditions can be readily satisfied and the recirculating gas stream can be subjected to catalysis only to the optimum extent.

EXAMPLE The following example illustrates the application of the invention to a specific set of circumstances and illustrates the considerations discussed above with relation to the amount of catalyst required, the amount of heat recovered, and volume of gases oxidized.

In an oven operated at a temperature of 470 F. and exhausting to the atmosphere at a rate of 3,200 S. C. F. M. the amount of solvent volatilized is assumed to be 15 gallons per hour or 0.25 gallon per minute. The oven atmosphere is recirculated at the rate of 32,000 S. C. F. M. of which 3,200 S. C. F. M. are by-passed in the recirculation system through the catalyst. A rate of volatilization of about 0.25 gallon per minute will yield under these circumstances an oven atmosphere containing a concentration of solvents within the generally accepted safe operating range. This oven is assumed to have heat losses of 2,000,000 B. t. u. per hour divided as follows: material losses', 300,000 B. t. u. per hour, radiation losses 300,000 B. t. u. per hour, and exhaust loss 1,400,000 B. t. u. per hour.

The solvents volatilized, 15 gallons per hour, are capable of providing approximately 120,000'15. t. u. per gallon or 1,800,000 B. t. u. per hour to the oven if they are completely oxidized. Where 50% Vof this. available heat is recovered, the heat supplied by these solvents will approximate 900,000 B. t. u. per hour. Fig. 3 shows that this amount of heat can be .recovered where the concentration lof solvents in the recirculating gases is equal to the concentration of solvents lin the exhausted gases and. the volume of gasescatalyzed per unit time (3,200

S. C. F. M.) is approximately equal to the volume exhausted per unit time. Where these `conditions obtain then, it is necessary to provide an additional 1,100,000 B. t. u. per hour from extraneous sour-ces such as the burners 20 and 34 of the embodiment of Fig. 1. In the operation ofan :oven of this type, of the 1,100,000 B. t. u. per hour provided from extraneous sources, most of this heat would be imparted by the burner 34 sin-ce in heating a lrelatively small volume of gases to catalytic oxidation temperatures relatively large quantities of extraneous heat would be needed. The additional heat supplied would be providedthrough the burner 20 and would be varied somewhat in accordance with .minor variations in oven 1operating conditions as sensed by thermocouple 24 and the fuel input would be correspondingly changed 'by controller 22. In normalY operation the oven willv have the following heat balance:

Under the foregoing conditions of operation the bypassed gases liowing in conduit 32 will be heated to about 975 F. while the main `stream of recirculating gases will be heated by about 8 F. to 478 F. The temperature of the resultant stream (which is reintroduced into the oven) will then be about 528 F. which is suitable for introduction into the oven.

The above example illustrates the fact that under ordinary operating conditions by oxidizing -a volume of gases equal to the volume exhausted from the oven approximately fifty percent of `the heat available in the solvents can be recovered and that under ordinary operating conditions thisheat will provide almost one-half of the total heat required to maintain the Ioven `at its operating temperature. It is therefore apparent then that substantial fuel savings can be effected by the practice of the invention. The .above example also illustrates that while a total of 1,800,000 B. t. u. per hour might theoretically be recovered from the vaporized solvent, it would be necessary to pass an enormous volume of gases over the catalyst in order to recover this much heat. This would be obviously impractical because 'it would be necessary to provide an enormous amount of pre-heat and the total amount of heat supplied by oxidation of vapors plus preheat would far exceed the requirements of the oven. Furthermore, the amount of catalyst required would render the process highly uneconomic. For example, to recover 1,620,000 B. t. u. per hour of the available 1,800,000 would require that the amount of gases passed over the oxidation catalyst per unit length of time be equal to nine times the volume of gases exhausted per unit length of time. Furthermore, it would be necessary to provide about 8,100,000 B. t. u./ hr. in preheat which would greatly exceed the requirements of the oven (2,000,000 B. t. u./ hn).

lParticular attention is -called to the fact that the invention can be incorporated into existing drying ovens with only slight structural modiications thereof and without disturbing the ovens previously existing heat balance. The embodiment shown in Figs. 1 land 2 constitutes a conventional type of existing oven with the by-pass conduit 32, secondary burners 34, and catalyst 40 added thereto. The heat supplie-d 'by the catalyst and the burners 20 and 34 is equal to the heat which in conventional operation would be supplied entirely by the primary burners 20.

The present invention is not limited to a particular catalyst and the principles thereof are applicable to virtually Iall suitable oxidation catalysts. However, a particular catalyst which will yield satisfactory results consists of a unit 40 of the type shown in Fig. 4 to consist of a pair of end plates 46 which are rigidly secured to each other in spaced relationship by a spacer bar 48. A plurality of rod-like elements 50 are supported between end plates 46 in apertures 52 provided on the opposing faces 'of the end plates. At least one end of each of the rod-like elements 50 is freely mounted in its aperture 52 for the purpose of providing suicient clearance to permit thermal expansion and contraction without stressing or fracture. The unit 40 is preferably constructed of a ceramic material such as high quality porcelain of the grade used in the manufacture of spark plugs. The Todlike elements 50 `are supericially coated with catalytic alumina, beryllia, zirconia or a suitable mixture of such materials, which is impregnated with about one percent to two percent platinum or other catalytically active metal based on theweight of the alumina coating. Catalysts of this type are described more fully in the co-pending application of Eugene J. Houdry, Serial No. 312,152, filed September 29, 1952 for Catalytic Structure and Composition.

While I have disclosed the invention with reference to the particular apparatus set up of Figs. l and 2, it is obvious that the invention might be practiced with drying ovens of other types. For example, the gases exhausted t-o lthe atmosphere might be extracted from the recirculating system rather than from the oven chamber vdirectly as in the `disclosed embodiment. If the exhaust gases were extracted from the recirculating system it would, of course, be preferable to provide an exhaust duct to the duct 16 upstream from the burners 20 since the gases exhausted would not be heated by the burner. It should also be noted that the invention is applicable to ovens which might be planned with the principles of the invention in mind. Where the principles of the invention are applied to an oven constructed specifically `for the practice of the invention, the primary burner might be omitted and the secondary burner relied upon to supply all of the heat required in excess of the amount liberated by catalytic oxidation. With this arrangement the tem-v perature sensing means for the preheat burner 34 would be located in the oven chamber rather than in the 'bypass conduit.

Other modifications within the scope of the appended claims will be obvious to those skilled in the art.

I claim:

1. In a drying oven of the type wherein an oxidizable mixture of air and solvent vapors is evolved during the drying process, said oven comprising an oven chamber, a recirculation duct and heating means in said duct, said duct and said heating means being operative to continuously withdraw a .stream of air-vapor mixture from said oven, raise the temperature of said stream and return said stream to the oven chamber, the improvement comprising: a by-pass duct leading from said recirculation duct at a point upstream from said heating means and leading into said recirculation duct at a point downstream from said heating means, an oxidation catalyst disposed within said by-pass duct, whereby a portion of said stream ilows through said by-pass duct, across said oxidation catalyst, and is subsequently returned to said stream, said oxidation catalyst being operative to raise the temperature of said portion by oxidation of the combustible vapors therein and thereby provide a substantial portion of the heat required to maintain said oven at its operating temperature, control means for said heating means, said control means being operative to control said heating means in such manner as to provide the additional heat required to maintain said oven at its operating temperature.

2. In a drying oven of the type wherein an oxifdizable mixture of air and solvent vapors is evolved during the `dryinU process, .said oven comprising an oven chamber, a recirculation duct and a first heating means in said duct, said duct and sai-d first heating means being operative to continuously withdraw a stream of air-vapor mixture from said oven chamber, raise the temperature of said stream and return said stream to said oven chamber, the improvement comprising a by-pass duct leading from said recirculation duct at a point upstream from said first heating means and leading into said recirculation duct at a point `downstream from said first heating means, a secondary heating means and an oxidation catalyst in said bypass duct, said catalyst being ydisposed downstream from said secondary heating means, said secondary heating means being operative to heat a stream of air-vapor mixture flowing in said by-pass duct by at least an amount suicient to render said stream amenable to `catalytic oxidation, said'catalyst being operative to oxidize the oxidizable vapors of said stream and thereby further raise the temperature thereof whereby a substantial portion of the vheat required to maintain .said oven at its operating avances 11 temperature is supplied by said secondary heating means and said catalytic oxidation.

3. In a drying oven of the type wherein an oxidizable mixture of air and solvent vapors is evolved during the drying process, said oven comprising an oven chamber, a recirculation duct and heating means in said duct, said duct and said heating means being operative to continuously withdraw a stream of air-vapor mixture from said oven, `raise the temperature of said stream and return said stream to the oven chamber, the improvement comprising: a oy-pass duct leading from .said recirculation duct at a point upstream from said heating means and leading into said recirculation duct at a point downstream from said heating means, an oxidation catalyst disposed Within said by-pass duct, whereby a portion oi' said stream flows through said by-pass duct, across said oxidation catalyst, and is subsequently returned to said stream, said oxidation catalyst being operative to raise the temperature of said portion by oxidation of the combustible vapors therein and thereby provide a substantial portion of the heat required to maintain said oven at its operating temperature, .said heating means being operative to supply the additional heat required to maintain said oven at its operating temperature.

4. In a drying oven of the type wherein an oxidizable mixture of air and solvent vapors is evolved during the drying process, said oven comprising an oven chamber, a recirculation duet and a rst heating means in rsaid duct, said duct and said iirst heating means being operative to continuously withdraw a stream of air-vapor mixture from said oven chamber, raise the temperature of said stream and return said stream to said oven chamber, the improvement comprising a by-pass duct leading from said recirculation duct at a point upstream from said first heating means and lea-ding into said recirculation duct at a point downstream from said rst heating means, a .second heating means and an oxidation catalyst in said by-pass duct, said catalyst being disposed. downstream from said second heating means, control means for said second heating means operative to control said second heating means in ing means and an oxidation catalyst in said by-pass duct, are heated by a relatively constant amount by the cumulative eiect of said second heating means and said catalyst to a temperature substantially in excess of the operating temperature of said oven whereby a substantial portion of the heat required to maintain said oven at its operating temperature is supplied by said second heating means. and by catalytic oxidation of the oxidizable components of said oxidizable mixture.

5. in a :drying oven of the type wherein an oxidizable mixture of air and solvent vapors is evolved during the drying process, said oven comprising an oven chamber, a recirculation duct and a first heating means in said duct, said duct and said first heating means being operative to continuously withdraw a stream of air-vapor mixture from said oven, raise the temperature of said stream and return said stream to the oven chamber, the improvement comprising: a by-pass duct leading from said recirculation duct at a point upstream from said first heating means and leading into said recirculation duct at a point downstream from said heating means, a second heating means and an oxidation catalyst disposed within said bypass duct, said catalyst being disposed downstream from said second heating means, control means for said second heating means operative to control said second heating means Vin such manner that gases owing through said bypass .duct are heated by a relatively constant amount by the cumulative elfect of said second heating means and said catalyst to a temperature substantially in excess of the operating temperature of said oven to thereby provide a substantial portion of the heat required to maintain said oven at its operating temperature, said first heating means being operative to supply the additional heat required to maintain said oven at its operating temperature, and control means for said irst heating means operative to control said tirstheating means in such manner as to supply the heat requirements of said oven which are in excess of the heat supplied by said second heating means and the catalytic oxidation of said vapors.

6. A method for the operation of a rccirculating type drying oven wherein combustible vapors are evolved which intermix with air within the oven chamber comprising the steps of continuously withdrawing a stream of vapor containing air from the oven chamber, heating said stream and recirculating it to the oven chamber, continuously exhausting a stream of vapor-containing air from the oven chamber to the atmosphere at a temperature substantially equivalent to the temperature of the stream withdrawn from said oven chamber for passage through said recirculation duc-t and at a rate suliicient to maintain the vapor concentration in said oven at a safe level, the rate of recirculation being high relative to the rate of exhaust to the atmosphere, dividing said recirculating stream into a maior and. a minor portion, preheating said minor portion to an elevated temperature at which catalytic oxidation of its vapor content effectively takes place7 passing said preheated minor portion in contact with oxidation catalyst to eect catalytic oxidation of its vapor content and to further increase its temperature, ecombining the thus heated minor portion with said major portion to produce a mixture at a temperature suitable for reintroduction into the oven chamber, and adjusting the rate of ilow 'of the minor portion of said recirculating stream over said catalyst with respect to ther rate of exhaust of said air-vapor mixture from said oven chamber to the atmosphere such that the ratio of i'low of the catalyzed stream to the exhaust stream does not exceed 3:1 whereby a substantial portion of the theoretically recoverable heat of said vapors is recovered for utilization in said oven with the use of a relatively small amount of oxidation catalyst.

7. ln a drying oven of the type wherein an oxidizable mixture of air and solvent vapors is evolved during the drying process, said oven comprising an oven chamber and a recirculation duct having an inlet end for withdrawing an air-vapor mixture from said oven chamber and an outlet end for returning heated gases to said oven chamber, the improvement comprising: a by-pass duct having its inlet and outlet connected into said recirculation duct between the inlet and outlet ends thereof, oxidation catalyst disposed within Said by-pass duct, heating means disposed within said by-pass duct upstream from said oxidation catalyst to heat the air-vapor mixture passing through said duct to a temperature suiiicient to render it amenable to catalytic oxidation, means for regulating the iiow through said recirculation duct and said' by-pass duct such that a minor portion of the gas mixture entering said recirculation duct from said oven chamber is withdrawn therefrom, ows through said bypass duct, is heated by said heating means, passes in contact with said catalyst whereby its temperature is further raised through catalytic oxidation of the combustible vapors present therein and is returned to said recirculation duct and'mixed with the gases flowing therein, the flow through said by-pass duct being controlled such that the heat supplied by said heating means and the catalytic oxidation of'said combustible vapors supplies at least a substantial portion of the heat required to maintain said oven at its operatingy temperature, and means for exhausting .rom the system to the atmosphere a stream of air-vapor mixture at substantially the same temperature as the stream entering said recirculation duct to maintain the vapor concentration in said oven at a safe level.

References Cited in the tile of this patent 

6. A METHOD FOR THE OPERATION OF A RECIRCULATING TYPE DRYING OVEN WHEREIN COMBUSTIBLE VAPORS ARE EVOLVED WHICH INTERMIX WITH AIR WITHIN THE OVEN CHAMBER COMPRISING THE STEPS OF CONTINUOUSLY WITHDRAWING A STREAM OF VAPOR CONTAINING AIR FROM THE OVEN CHAMBER, HEATING SAID STREAM AND RECIRCULATING IT TO THE OVEN CHAMBER, CONTINUOUSLY EXHAUSTING A STREAM OF VAPOR-CONTAINING AIR FROM THE OVEN CHAMBER TO THE ATMOSPHERE AT A TEMPERATURE SUBSTANTIALLY EQUIVALENT TO THE TEMPERATURE OF THE STREAM WITHDRAWN FROM SAID OVEN CHAMBER FOR PASSAGE THROUGH SAID RECIRCULATION DUCT AND AT A RATE SUFFICIENT TO MAINTAIN THE VAPOR CONCENTRATION IN SAID OVEN AT A SAFE LEVEL, THE RATE OF CIRCULATION BEING HIGH RELATIVE TO THE RATE OF EXHAUST TO THE ATMOSPHERE, DIVIDING SAID RECIRCULATING STREAM INTO A MAJOR AND A MINOR PORTION, PREHEATING SAID MINOR PORTION TO AN ELEVATED TEMPERATURE AT WHICH CATALYTIC OXIDATION OF ITS VAPOR CONTENT EFFECTIVELY TAKES PLACE, PASSING SAID PREHEATED MINOR PORTION IN CONTACT WITH OXIDATION CATALYST TO EFFCT CATALYTIC OXIDATION OF ITS VAPOR CONTENT AND TO FURTHER INCREASE ITS TEMPERATURE, RECOMBINING THE THUS HEATED MINOR PORTION WITH SAID MAJOR PORTION TO PRODUCE A MIXTURE AT A TEMPERATURE SUITABLE FOR REINPRODUCTION INTO THE OVEN CHAMBER, AND ADJUSTING THE RATE OF FLOW OF THE MINOR PORTION OF SAID RECIRCULATING STREAM OVER SAID CATALYST WITH RESPECT TO THE RATE OF EXHAUST OF SAID AIR-VAPOR MIXTURE FROM SAID OVEN CHAMBER TO THE ATMOSPHERE SUCH THAT THE RATIO OF FLOW OF THE CATALYZED STREAM TO THE EXHAUST STREAM DOES NOT EXCEED 3:1 WHEREBY A SUBSTANTIAL PORTION OF THE THEORETICALLY RECOVERABLE HEAT OF SAID VAPORS IS RECOVERED FOR UTILIZATION IN SAID OVEN WITH THE USE OF A RELATIVELY SMALL AMOUNT OF OXIDATION CATALYST. 